Patent Literature (PTL) 1 discloses a detection device which achieves detection as to whether a human hand is in contact with a steering wheel, taking advantage of characteristics of an oscillator circuit in that the frequency of the oscillator circuit changes as capacitance changes.
Meanwhile, PTL 2 discloses a vehicle seat apparatus which makes use of a seat heater to detect whether a passenger is being seated. The vehicle seat apparatus serves as a seat heater and also as an electrode for detecting whether the passenger is being seated. Thus, an oscillator circuit is obviated, and cost is reduced.
FIG. 12 is a schematic sectional view of a vehicle seat apparatus in a conventional technique (see PTL 2).
In the figure, seat heater 8 is included in a seat of a vehicle seat apparatus 1, and connected via coupling capacitor 22 to seating detection circuit 14. Seat heater 8 is supplied with power via seat-heater release switches 11 and 12. Seating detection circuit 14 detects passenger's action of seating himself/herself on the vehicle seat apparatus 1, when seat-heater release switches 11 and 12 are off. Specifically, seating detection circuit 14 includes an alternating-current (AC) power source, a band-pass filter, a detection-and-smoothing circuit, an amplifier circuit, an analog-to-digital converter, and a microcomputer. The band-pass filter can be configured with a resonant filter which passes a predetermined frequency value output by the AC power source. An alternating voltage component extracted from the band-pass filter is detected and smoothed by the detection-and-smoothing circuit, increased by the amplifier circuit to predetermined amplitude, converted by the analog-to-digital converter into a digital signal, and then loaded into the microcomputer. The microcomputer determines that a passenger is being seated if the digital signal loaded is less than a predetermined threshold.
As such, PTL 2 discloses making use of a heater wire as one electrode of capacitance for detecting whether the passenger is being seated. Here, to use the heater wire as an electrode for the detection, the electrode need be floating, and thus seat-heater release switches 11 and 12 are provided to isolate seat heater 8 from GND (a ground level). Typically, seat-heater release switches 11 and 12 are on, that is, in an energized state, when a heater wire is warming, and are off, that is, in a blocked state, when seating detection circuit 14 is in operation.